I poured a circular 2 inch thick slab of concrete with the intention of going 4 inches thick for a fire pit when there was an issue that took me away from the project. can I go back and pour another layer over the cured concrete?

Sure but if its only in the fire pit area, why not cement in fire brick on bottom to protect the cement? Basic cement is not designed for heat of coals, fire brick is.

If you decide to go the extra cement route, be sure that the older cement is clean, ideally rough on the surface so it bonds but other then that you should have no issue.

If its structural in any way, then you should remove the cement and start over however.